1. Field of the Invention
The present invention relates to an image forming device employing an electrophotographic system, more particularly, to an image forming device capable of supplying a lubricant to an image carrier.
2. Description of the Related Art
In recent years, electrophotographic image forming devices have been demanded to improve image quality with respect to high resolution and photographic image reproduction, for example. As a powerful means for meeting the demand, a method for making toner particles smaller in diameter or spherical has been used.
However, when toner particles are made smaller in diameter or spherical, the toner is liable to pass through the gap between a cleaning blade and a photoconductor. Making the toner particles smaller in diameter increases an adhesion force associated with the Van der Waals force acting between the toner and the photoconductor (image carrier). Furthermore, when the toner particles are made smaller in diameter, the toner is liable to enter the portion (nip portion) between the photoconductor and the cleaning blade. Moreover, when the toner particles are made spherical, the toner is liable to roll at the portion between the photoconductor and the cleaning blade. Therefore, the toner is liable to enter the nip portion by rolling.
If the toner passes through the cleaning blade, the toner remains on the photoconductor. If the remaining toner is transferred together with the next image, image noise, such as black lines, is generated. If the remaining toner blocks the light used for exposure, a portion in which no latent image is formed is generated on the photoconductor.
Therefore, for the purpose of facilitating the cleaning of the toner, technologies have been proposed in which a substance for lowering the friction coefficient of the photoconductor is supplied onto the photoconductor. In these technologies, for example, a lubricant made of a metal salt of a fatty acid, such as zinc stearate, is supplied to the surface of the photoconductor. When the lubricant is supplied onto the photoconductor, the attachment force and the friction force of the toner to the photoconductor are lowered. As a result, the toner can be sufficiently removed even when the cleaning blade or the like is used.
(Lubricant Development Supply System)
A lubricant development supply system is available as a system for supplying a lubricant to the photoconductor. In the lubricant development supply system, a lubricant in a powder form is added to the outside of the toner. When development is performed using the toner, the lubricant is supplied to the photoconductor, and the lubricant in a powder form is formed into a thin film on the photoconductor using a cleaning blade.
Since the lubricant development supply system does not require special components for supplying the lubricant, the system is very advantageous in cost and space. For this reason, the system is adopted for many image forming devices operating in low and middle speed ranges.
However, when the toner to which the lubricant powder is added is stirred inside a developing unit, friction occurs between the toner and the lubricant powder, whereby not only the toner but also the lubricant powder is charged in most cases. In the case that the lubricant powder is charged in the same polarity as the polarity of the toner, the lubricant powder attaches to the image area of the photoconductor. In the case that the lubricant powder is charged in the polarity opposite to that of the toner, the lubricant powder attaches to the non-image area of the photoconductor. Consequently, when the same image is printed continuously or for a long period, portions having different friction coefficients are generated on the surface of the photoconductor. In addition, in the case that a low density image is printed for a long period, the supply amount itself of the lubricant to the cleaning blade decreases. In this case, the friction coefficient on the surface of the photoconductor does not lower sufficiently. Furthermore, when the amount of the lubricant powder to be added to the toner is increased to increase the supply amount of the lubricant, the lubricant powder is liable to be transferred to a toner carrier inside the developing unit. When the lubricant powder is transferred to the carrier, the charge characteristic of the toner is lost, and reversely charged toner is generated. As a result, the toner is developed in the non-image area. In other words, a phenomenon referred to as fogging occurs. Furthermore, when the lubricant is applied to the developing roller of the developing unit, the friction coefficient of the developing roller is lowered. As a result, the toner is not conveyed properly, and the printing image density of a high density image is lowered.
(Lubricant Application System)
As another system for supplying a lubricant to the photoconductor, a lubricant application system is available. In this lubricant application system, a solid lubricant is pressed to a rotatable brush (rotation member), and lubricant powder scraped off with the brush is applied to the photoconductor.
The lubricant application system requires, in addition to a cleaning blade, a roll-shaped brush for scraping off lubricant powder and supplying the scraped lubricant powder to the photoconductor, a solid lubricant, components for holding these, a spring for pressing these, etc. Therefore, the lubricant application system is high in cost, and a large space is required. However, since the lubricant application system can actively apply the lubricant to the photoconductor, the stability of the application is relatively high, and the environmental dependency of the system is small. Furthermore, in the case that the brush is disposed on the upstream side of the cleaning blade, the cleaning ability of the cleaning blade can be enhanced further by actively removing the toner using the brush.
However, when the rotating brush applies the lubricant to the photoconductor, the toner gradually attaches to the surface of the bristles of the brush. Since the toner having attached thereto has a roller action, the brush is difficult to scrape off the solid lubricant. As a result, a required amount of the lubricant powder cannot be supplied stably onto the photoconductor.
Therefore, in order that the toner having attached to the brush can be removed, a toner removing member referred to as a flicker is disposed so that the bristles of the brush make contact therewith. However, when the flicker makes contact with the brush, stress is applied to the fibers of the brush, whereby the fibers are worn or deflected. As a result, the ability of the brush for scraping off the solid lubricant is lowered. In addition, for the purpose of uniformly applying the lubricant to the photoconductor, it is necessary to raise the density of the fibers of the application brush. However, if the density of the fibers is raised, it becomes difficult to remove the toner remaining inside the application brush. As a result, a problem referred to as a toner rolling phenomenon occurs. In other words, the brush is stiffened with the toner and formed into a roller shape. Furthermore, when the density of the fibers is raised, the stiffness of the brush (the stiffness of the bristles) increases. As a result, the surface layer of the photoconductor is damaged or the abrasion of the photoconductor is accelerated.
Still further, in the case that the lubricant application system is applied to an image forming device operating in a high speed range, the capacity of a process unit increases. In the image forming device operating in the high speed range, the service life of the unit is set so as to be relatively long. Therefore, the size of the solid lubricant is required to be increased so that the lubricant can be supplied for a long period. The solid lubricant is also disposed inside the unit. Consequently, the capacity of the unit is increased by increasing the size of the solid lubricant.
In the lubricant development supply system, the toner makes friction contact with the lubricant powder, and the lubricant powder is charged as described above. Therefore, the lubricant powder attaches unevenly to one of the image area and the non-image area. As a result, the distribution of the lubricant on the surface of the photoconductor becomes uneven. On the other hand, even in the lubricant application system, when the lubricant powder is scraped off with the brush from the solid lubricant, the brush makes friction contact with the lubricant powder. As a result, the lubricant powder is charged. The inventors of the present invention have recognized this fact by carrying out experiments. Therefore, even in the lubricant application system, the distribution of the lubricant on the surface of the photoconductor becomes uneven.
Furthermore, in the lubricant application system, since the brush is stained with the toner, the lubricant cannot be supplied to the surface of the photoconductor for a long period. Technologies for solving the problem in which the brush is stained with the toner have been described in Patent Document 1 and Patent Document 2. Patent Document 1 is Japanese Patent Application Laid-open Publication No. 2006-251751, and Patent Document 2 is Japanese Patent Application Laid-open Publication No. 2007-310336.
In the system proposed in Patent Document 1, a solid lubricant is applied on the downstream side of a cleaning blade, and a rubber blade, that is, a so-called leveling blade, is made contact with a photoconductor on the downstream side of the solid lubricant, thereby making the lubricant into a thin film.
With this system, after toner is removed from the photoconductor, an application brush makes contact with the photoconductor. Therefore, it is assumed that the application brush is not stained with the toner and that the surface of the photoconductor is maintained in a state in which the friction coefficient thereof is stably low by virtue of a relatively small amount of the lubricant.
However, this system has the following problems. If the lubricant is not formed into a film by the leveling blade but remains in a particulate state and passes through the leveling blade, the lubricant powder stains a charging unit disposed on the downstream side of the leveling blade. As a result, an improper image is generated. For the purpose of avoiding the generation of such an improper image, the contact force of the leveling blade is required to be set so as to be equal to or higher than the contact force of the cleaning blade. In this case, the friction force generated by the cleaning blade and the friction force generated by the leveling blade are added to the photoconductor. As a result, the load to a drive motor increases. Furthermore, since the leveling blade is installed additionally, it is necessary to secure a space for accommodating the leveling blade inside a process unit. In the case of a unit equipped with a photoconductor having a relatively small diameter, there is a restriction in the placement of the blade. Therefore, it is necessary to make the other electrophotographic process units, such as a charging unit, compact in size. As a result, the cost of the image forming device according to the system increases.
In the system proposed in Patent Document 2, an application brush is disposed so as not to make contact with a photoconductor, and a lubricant is charged by applying a potential to a flicker, whereby the lubricant is attached to the photoconductor by virtue of the electric field formed between the brush and the photoconductor.
In the case of this system, since the brush is disposed so as not to make contact with the photoconductor, the brush is prevented from being stained with toner directly. However, since the brush does not make contact with the photoconductor, it is necessary to scrape off a large amount of the lubricant with the brush so that the lubricant is attached securely to the photoconductor. Therefore, an excessive amount of the lubricant is consumed, and it is necessary to increase the size of the lubricant. As a result, the cost of the image forming device according to this system increases.